A broad investor retreat has pushed the market for digital currencies down 70% from its January high, reflecting user frustration over their modest inroads into commerce and a general shakeout in speculative investments.

The value of all cryptocurrencies in circulation this week fell below $200 billion for the first time in 2018, its lowest since November. The selling has been widespread and, some holders say, indiscriminate. Of the top 100 cryptocurrencies by market value, 98 were down over 24 hours, according to research...

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